Bill Gurstelle Teaches Science With Trebuchets

Bill Gurstelle, inventor and author of Absinthe and Flamethrowers and Woosh Boom Splat, ditched his job to tinker full-time. Today, he focuses on launching fiery projectiles and high-velocity vegetables in the name of scientific instruction.

On a warm late-summer evening, a crowd gathers in the parking lot of the Susan B. Anthony Middle School in Minneapolis, Minn., to watch a man in a navy blue jumpsuit as he prepares to fire his catapult. He pivots the 800-pound machine by its fir and oak beams, aiming it at a grassy slope about 80 yards away. Passing dog walkers pause, kids lean bikes on kickstands, and a security guard strolls over from the school to get a closer look.

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"We'll be doing some flinging before long," says the man in the jumpsuit, William Gurstelle, the catapult's builder. On one side of the machine's central fulcrum hangs a counterweight filled with 400 pounds of rocks and lead shot. On the other side of the pivot is an elegantly tapered 5-foot-long throwing arm pointing skyward. Gurstelle grabs the tip of the arm and pulls it down nearly 180 degrees until it's almost perpendicular to the asphalt, then latches it to the catapult's base. The 400-pound weight rises in opposition, poised like a boot ready to kick. Gurstelle gathers the ammunition: baseballs impaled by eyebolts, a box of gym socks and a bottle of lighter fluid. He drops a ball into a sock, hooks it to a sling at the throwing arm's tip, soaks the stuffed sock in accelerant and sets it all on fire.

Gurstelle steps back as the flames flicker and tightens his grip on a 6-foot rope tied to the latch pinning the throwing arm. He counts to three and yanks the line. The latch springs, the weight drops, the catapult creaks, and the throwing arm rises like a basketball player making a hook shot. The ball slings upward so fast that the flames seem to disengage. But the stocking stays ablaze, a low-level comet in a sizzling line drive. It hits the grassy hill with a thunk, ricocheting embers of sock elastic, and rolls down to the parking lot. "Is that a tray-boo-chit?" a bystander asks. "Correct," Gurstelle says, "except that it's treb-yoo-SHAY."

Gurstelle seizes any opportunity to educate his audience. He explains how to build a model of this particular trebuchet in his third book, The Art of the Catapult, with copious details on its role in the siege of Scotland's Stirling Castle in 1304, where it was known as Ludgar the War Wolf. Gurstelle's sixth book, 2009's Absinthe and Flamethrowers, balances smoke-bomb recipes with studies suggesting that a reasonable amount of risk-taking can make you a happier person. "I find a certain nobility in living a little more dangerously than the average guy," Gurstelle says.

Gurstelle got his first taste of danger during downhill slaloms on soapbox derby racers that he and his grandfather built using broken chairs and scrap lumber from the family's house in St. Paul. Gurstelle says his grandfather was a "bigger hammer" type of guy--construction snafus can all be solved, the thinking goes, by pounding the project with a bigger hammer. Lessons in precision came from Gurstelle's pharmacist father, who stocked Gurstelle's basement chemistry sets with vats of hydrochloric acid and metallic zinc. Gurstelle soon began modifying equipment, bending glassware into alembics and pot stills. Before long, he was cooking up his own gunpowder.

Despite Gurstelle's prowess his work rankled his teachers. "I was always told I had great potential but wasn't living up to it," he says. He struggled to find his niche, even after earning a degree in mechanical engineering from the University of Wisconsin. Working as a manufacturing engineer, he was bored welding tiny electrodes to other tiny electrodes. He left for a job designing industrial ovens and furnaces. "Grinding, punching, shearing, making ovens big enough to dip a Toyota into. It's what all American industry should be like," he says. But he lost interest in continuously redesigning the same oven. So after a misguided detour into an MBA program--"I thought I'd be this captain of industry," he says--Gurstelle wound up with a gig as a billing analyst for Northwestern Bell. "Who could be happy doing that?" he asks.

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Tinkering after hours during those dreary years, Gurstelle compiled test notes and performance data on the devices he built in his spare time--potato cannons fueled by hair spray, bratwurst missiles launched on bursts of compressed air or ghostly floating orbs made of plastic dry-cleaner bags and Sterno cans. The plans for 13 projects eventually became the manuscript for Backyard Ballistics, his first book. After its publication in 2001, Gurstelle quit the telecommunications industry. "And I never looked back," he says.

He found satisfaction as a full-time writer publishing plans for his projects. "Having control over the physical environment lets me parse it out and see what's inside it," Gurstelle says. After demonstrating his creations, he encourages his fans to go forth and tinker. "Not to be too highfalutin about it, but I think there's something good and noble in getting people interested in finding out what they can do," he says.

Sometimes Gurstelle hears about the results. A Backyard Ballistics reader once wrote to tell him that she still has the first spud gun she built, a family collaboration executed per Gurstelle's instructions. "It is one of the strongest memories I have of my dad and me together before he passed away. Thank you," she wrote.

The day after the catapult launch, the scent of peeled potatoes permeates Gurstelle's shop. Inside the concrete-walled room on an alley behind his home, a mesh sack of russets shares space on the workbench with PVC scraps, tees, purple primer and a Stanley SharpTooth crosscut handsaw. Labeled bins along each wall hold grinding wheels, copper tubing, rope, clothespins, casters, measuring cups and muffin pans. "A clean workbench is a happy workbench," Gurstelle says, sweeping the plastic plumbing pieces into their respective containers. Gurstelle leaves a dog-eared copy of the Mechanical Engineering Reference Manual lying open on the table, turned to a page of uniform acceleration formulas. Next to it, a stack of graph paper covered with scrawled equations and computations contains the lab notes on a supersize spud gun.

Gurstelle wants to see those notes in a science-class lesson plan on particle kinetics. "The potato only goes in two directions--up and down--but you can tell a lot of stuff from that," he says. "Acceleration, muzzle velocity, altitude of apogee--I like to know these things." He has received one letter in support of his vision, from a California high school senior who used Backyard Ballistics to co-teach an elective course. "I want you to know that the students in the class are both having fun and effectively learning science," he wrote. "Thanks for making such great experiments available to all the amateur scientists of the world!"

Out in the alley, Gurstelle sets up his latest cannon--82 inches of PVC pipe emblazoned with stickers that read, "Noisy Plumber." The gun is mounted vertically on a plywood stand, steadied on the concrete by 10-pound barbell plates. At the coupling where the gun's barrel meets its combustion chamber, purple stains of PVC primer show beneath the hardened ooze of excess cement. A threaded end cap, skewered by a flint-and-steel lantern sparker, seals the chamber. This allows Gurstelle to ignite the cannon without touching a flame to the fuel--unscented Aqua Net.

Gurstelle approaches the gun with a potato and a broomstick. He twists the spud, forcing it into the barrel, and then uses the broomstick to tamp it down. With deft motions, Gurstelle unscrews the end cap, spritzes a cloud of hair spray into the combustion chamber, pops the lid on and spins the igniter.

Listening to the potato launch, it becomes clear how Gurstelle settled on the title of his fifth book, Whoosh Boom Splat. He watches the russet shrink to a pinprick against the blue sky while a stopwatch measures seconds aloft. A seeming eternity after the whoosh and boom, a neighbor's concrete patio gets the splat. Vibrations from the impact cause Gurstelle's deaf dog to bark wildly nearby. The engineer rushes to his notebook, plugging in variables to document the results of this 8.8-second flight: muzzle velocity, 96 mph; apogee, 315 feet. "I said 4.4 seconds, right?" Gurstelle asks. He double-checks the numbers. Problem solved. The Noisy Plumber is ready to debut in public education. All Gurstelle needs now is an audience.